1. Field of the Invention
The present disclosure relates to the preparation of diamond films, and in particular, to the methods of deposition of diamond on valve metal substrates.
2. Related Art
High quality diamond film has become an attractive electrode material in electrochemical research and in various applications. As an example, diamond film electrodes can sustain redox reactions with more positive or negative standard potentials than those of water split, such as those with large over potentials like the evolution of chlorine, oxygen and hydrogen gases. As another example, diamond film electrodes can support the degradation of various refractory pollutants with high current efficiency, such as ammonia, cyanide, phenol, chlorophenols, aniline, TCE, various dyes, surfactants, and landfill leachate. These applications are discussed in the following articles: R. Tenne et al, J. Electroanal. Chem. 347, 409, (1993); J. Iniesta et al, Electrochem. Commun. 3, 346, (2001); X. Chen et al, Environ. Sci. Technol. 37, 5021, (2003); I. Troster et al, Diamond Relat. Mater. 11, 640, (2002); X. Chen et al, Chem. Eng. Sci. 58, 995, (2003). Moreover, diamond electrodes provide a low and stable background current, which has led to their use in the electro-analytical field.
Methods for diamond film deposition on nondiamond substrates include hot-filament chemical vapor deposition (HFCVD) and microwave plasma assisted chemical vapor deposition (MWPACVD). For these known CVD methods, low pressure (13.3-133 mbar) and high substrate temperature (600-1000° C.) are typically necessary. Commonly, these CVD methods take place in a hydrogen gas environment, which includes gases such as hydrocarbon chemicals, especially methane gas. Hydrogen molecules are decomposed into atomic hydrogen under the high energy input to the CVD system, such as the thermal energy from the filament in HFCVD system and plasma energy in MWPACVD system. The hydrogen radicals play multiple and critical roles during the process of diamond formation, and work as the main reactants and the energy source for the reactions. Low pressure in the system prolongs the retention time and increases the mean free path of the radicals produced in the gas phase. The high substrate temperature provides the energy for the surface reaction to form diamond crystals.
Some methods are known for depositing diamond film on various nondiamond substrates, including titanium, with CVD methods, but these are all problematic. In the methods using a titanium substrate, due to the high substrate temperature and the large difference of the thermal expansion coefficient between diamond and titanium, the diamond and substrate experience nearly a 7 GPa thermal stress under a substrate temperature of 1170 K. Further, in these methods, intrinsic stress results from structural mismatches, such as different lattice constant between the deposited diamond film and the substrate. Moreover, residual stress in the film, resulting from thermal stress and intrinsic stress, will reduce adhesion of the diamond film to the substrate, and may even cause the diamond film to peel away from the substrate during the cooling process after deposition. Accordingly, stability of the diamond film electrode is of the highest concern, and has impeded its widespread utilization.
U.S. Patent Application Publication No. 2005/0186345 suggests a pre-treatment process to obtain a uniform and high density nucleus with good reproducibility. The process includes the consecutive processes of blasting with ceramic particles, surface cleaning with an acid or base wash, heat treatment in vacuum or inert atmosphere, and ultrasonic scratching in nanodiamond particles. It is believed that these pre-treatment steps allow the formation of interlayers between the film and substrate during coating of the substrate by CVD, PVD, sputtering, or plating methods. These interlayers ideally have only small thermal coefficient differences and lattice constant differences between themselves and their adjacent layers, and thus play a positive role in improving the adhesion of the diamond film with the substrate. However, this technique is problematic. Particularly, the final pretreatment process of ultrasonic scratching with high hardness particles can leave the coated interlayer removable. Loss of the interlayer leads to peeling of the superficial film, causing the electrode to stop functioning. Further, this technique does not account for the thickness and porosity of the interlayers being formed, where think and porous interlayers are known to reduce adhesion of film to substrate. Additionally, this technique only attends to the formation of an interlayer between the diamond film and substrate, and not to the formation of useful interlayers of varying degrees of diamond content.
U.S. Pat. No. 5,587,013 is drawn to a technique for reducing residual stress between a diamond film and substrate, but this technique too is problematic. The technique involves alternately growing a potential-concave diamond layer (substrate temperature of 880° C. to 950° C. and a hydrocarbon ratio of 2.5 vol % to 3.5 vol %) and a potential-convex diamond layer (substrate temperature of 800° C. to 850° C. and a hydrocarbon ration of 0.5 vol % to 1.5 vol %). This technique fails to improve adhesion at the substrate surface, ignores the formation and nature of any interlayers, and further provides poor superficial diamond quality when the substrate temperature is reduced.
Similarly, U.S. Pat. No. 6,319,439 is drawn to a technique for compensating the intrinsic tensile stress of a diamond film with a step down control of the deposition temperature during deposition. Again, this technique fails to improve adhesion at the substrate surface, ignores the formation and nature of any interlayers, and further provides poor superficial diamond quality when the substrate temperature is reduced.
Thus, all of the known methods for vapor deposition of diamond films fail to provide both a superior superficial diamond quality and a strong adhesion to an underlying substrate.